Fixed test scripts to use binaries from usr_bin_dir instead of code_root_dirs

Nova has moved to using entry points for nova binaries.
These are now located at /usr/local/bin/* instead of /nova_code_root/bin/*
Fixed the config file to contain a new entry for usr_bin_dir, from where it
should pick up these binaries. (default is /usr/local/bin)

Fixes bug 1187965

Change-Id: Ia0296e6d2d2d3eebb6cf46e958fe280c8fbfe42a